1. Field Of The Invention
The present invention relates to energy-saving electronic flash apparatus and, more particularly, to apparatus for signaling the readiness of the flash apparatus for an exposure operation.
2. Description Relative to the Prior Art
Many commercially available electronic flash units monitor the voltage on a flash-firing capacitor and, via a ready lamp or by enabling a shutter release mechanism in an associated camera, inform a camera operator when there is sufficient firing voltage for a flash exposure. In some electronic flash units, the firing voltage is considered to be sufficient when the capacitor is charged above a predetermined voltage level that corresponds to the minimum voltage required to ignite the flash tube. This arrangement has the advantage of permitting a rapid sequence of flash exposures but suffers the risk of underexposing subjects located beyond the flash distance corresponding to the minimum flash tube firing voltage.
In other electronic flash units, the firing voltage is considered to be sufficient when the flash-firing capacitor is charged above a predetermined voltage threshold that is a given percentage of the maximum operating voltage of the flash unit. Typically, the threshold voltage produces flash illumination that results in an underexposure of about one half to two thirds of a stop from the flash illumination that is produced when the capacitor is fully charged. Although this arrangement has the advantage of providing adequate flash illumination for subjects at the maximum flash distance, it suffers the disadvantage of preventing a more rapid sequence of exposures of subjects at the near end of the flash exposure range.
U.S. Pat. No. 4,258,991 discloses an electronic flash unit having a plurality of light emitting diodes (LED's) which successively and stepwise emit light as the charging voltage across the flash capacitor increases to corresponding predetermined voltage levels. Subject distances are marked in association with the LED's so that the distance corresponding to an LED that is energized is approximately the subject distance at which the operator can take an adequately exposed flash picture. Disadvantages of this arrangement are that (1) the operator must be an accurate judge of distance, and (2) if the photographic subject is located between the distances marked by two successive LED's, the operator usually waits for the flash unit to charge beyond the time that is necessary to provide adequate flash illumination.